In image formation, measures for further lowering a toner fixing temperature have been increasingly taken with an increase in a demand for energy saving. As one of the measures, it has been proposed to further lower the fixing temperature by the use of resin with a low softening temperature as a constituent material of toner. However, when using the resin with a low softening temperature, blocking in which toner particles adhere to each other sometimes occur when allowed to stand still during storage or transportation. Then, a core-shell structure has been proposed in which the resin with a low softening point is covered with resin with a high softening point (PTLs 1 to 3). It is considered that the use of toner particle having the core-shell structure (hereinafter also referred to as core-shell structure toner particles) allows manufacturing of toner in which the resin with a low softening point is not exposed to the toner surface and heat-resistant storageability and low-temperature fixability have been achieved. Moreover, in order to manufacture the toner particle having the core-shell structure, a method has been reported which includes controlling a change in the zeta potential of the core and the shell during manufacturing (PTL 4).
In recent years, there is a tendency such that the use of organic solvents is restricted from the position of environmental protection, saving resources, regulation of dangerous substances by the Fire Services Act, and workplace environment improvement. Thus, a development of manufacturing a fine particle water dispersion under the conditions of substantially not using a solvent has been actively performed (PTLs 5 to 7).
As a result of our examination, it was confirmed that when it was attempted to form a core-shell structure by using fine particles manufactured under the conditions of substantially not using a solvent as shell particles, and then attaching the shell particles to core particles in an aqueous medium, the shell particles are aggregated to each other. Therefore, in such a state, the core-shell structure is not formed or an aggregate of the shell particles adhere to the core particles, resulting in the formation of a nonuniform core-shell structure.
Even when a manufacturing method described in PTL 4 is used and even when a fine particle dispersion manufactured under the conditions of substantially not using a solvent is used as the shell particles, the aggregation of the shell particles occurs, so that the core-shell structure cannot be formed.
Thus, it has been difficult to form the core-shell structure using the fine particles manufactured under the conditions of substantially not using a solvent as the shell particles.